1, the entity: the active element of the n-layer is referred to as n layer entity. The same layer entity called peer entities.
2, the protocol: rules for network entities in the peer data exchange established standards or conventions called network protocol. 【Level】
· Syntax: transmission of predetermined data format inch
* Semantics: predetermined functions to be accomplished
Synchronous: a predetermined sequence of the various operations
3, an interface (service access point SAP): an upper layer using a lower inlet and services.
4, Service: lower to upper layer function provided by the adjacent call. 【vertical】